The one thing that is the most difficult thing to accomplish in art and photography is to emulate HOW the eyes see. If you adjust the aperture on a camera to blur the objects in front of and beyond the subject, you are getting a simulated perspective of the eye view only.

What the eye actually sees is EVERYTHING in focus, but concentrating on the subject at hand.

If you stare at an object, but remove the object from your mind’s focus, you will see the things in the background aren’t out of focus, but merely reduced in detail. Also, they are separated into two images, coming from each eye. One eye cannot adjust focus. The point where the eye direction meets brings the two images together at that point.

Imagine looking at a 3D movie without glasses. (the kind that separates the red and blue colors) That’s what the foreground and background looks like, but the colors are correct. The focus is on the middle-the subject, which is lined up by the eyes.

For an artist, a painting or photograph should be in focus totally, because the viewer’s eye will actually make the “other objects” “out of focus”. The realistic paintings from centuries ago are very detailed, even with the background items. Your brain knows where to look and knows what the subject is, due to lighting, placement, etc. That’s why these paintings are so enduring.

For a camera to capture what the eye sees, it would need 6 lenses to take 3 pictures at once. Two for foreground, two for subject and two for background.